2-DIMENSIONAL ELECTROPHORETIC ANALYSIS OF PROTEINS OF BOVINE MAMMARY-GLAND SECRETIONS COLLECTED DURING THE DRY PERIOD

Proteins in mammary gland secretions, collected from Holstein cows during the dry period, were analyzed by preparative isoelectric focusing, followed by SDS-PAGE. Protein profiles changed throughout the dry period. intact casein bands were present throughout the dry period, but in reduced proportions from d 7 after dry-off through d 7 prepartum. Breakdown fragments of casein were particularly apparent in secretions from d 7 to 21 of the dry. period. A fragment of beta-CN (approximately 13 kDa) was identified in secretions collected on d-1 prior to dry-off through d 21 of the dry period. Beta-Lactoglobulin generally focused in a limited number of fractions, but, in the sample at d 21 prepartum, beta-LG focused over a broad pH range. Lactoferrin was found in all fractions after isoelectric focusing, but specific degradation products of lactoferrin were apparent only at certain times during the dry period. This method is a valuable approach for separation of proteins from the complex mixtures found in mammary secretions during the dry period. A number of peptides identified by this method may be generated from breakdown of milk proteins in the mammary gland during the dry period and could have a role in the involuting gland.